Steering control device

ABSTRACT

A steering control device restrains not only a steered angle but also a steering angle from exceeding their upper limit values. A maximum value selection circuit selects a maximum value of a target steered angle and a target steering angle as an end angle and outputs the end angle to a limiting reaction force setting circuit. The limiting reaction force setting circuit increases a limiting reaction force rapidly if the end angle becomes equal to or larger than a common threshold. A target steering angle setting circuit sets a target steering angle based on torque applied to the steering minus the limiting reaction force etc. A steering angle control circuit sets reaction torque as a manipulated variable for feedback controlling the steering angle to the target steering angle. An operation signal generation circuit outputs an operation signal to a reaction force motor so as to achieve the reaction torque.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-146895 filed on Jul. 24, 2015 including the specification, drawings, and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to steering control devices that output an operation signal to a steering device that can change the steering angle ratio, namely the ratio between the steered angle of steered wheels of a vehicle and the steering angle, or the rotation angle of a steering.

2. Description of the Related Art

Conventionally, in steering devices, there is an upper limit value permitted for the steered angle of the steered wheels. For example, in those steering devices having a rack and pinion mechanism, axial displacement of a rack is restricted by a rack stopper (end stopper). There is also an upper limit value for the steering angle of the steering wheel in accordance with the upper limit value of the steered angle. Normally, the steered angle and the steering angle are positive when they are rotation angles in a predetermined direction, and are negative when they are rotation angles in the opposite direction. In the following description, the expression “the steering angle and the steered angle are large” means that a variation from the neutral position is large. In other words, the expression “the steering angle and the steered angle are large” means that the absolute values of the steering angle and the steered angle that can have positive and negative values are large.

A process in which a reaction force is applied to the steering wheel immediately before the absolute value of the steered angle reaches its upper limit value in order to control an increase in steered angle is well known in the art. This process is what is called an end abutting process (steering limiting process). For example, Japanese Pat. No. 4,725,132 describes a steering limiting process in a steer-by-wire system that steers the steered wheels by steering actuators in accordance with the operation of the steering wheel while cutting off power transmission between the steering wheel and the steered wheels.

The steering wheel is equipped with an airbag, a horn, etc. As well known in the art, electrical wiring for these components is connected via a spiral cable, a signal cable formed in a spiral shape, to electronic equipment fixed to a vehicle body. Japanese Patent Application Publication No. 2006-176102 (JP 2006-176102 A) describes that the length of a spiral cable is set so that a steering can be operated until a rack shaft contacts a rack stopper when the steering angle ratio, namely the ratio of the steered angle to the steering angle, is minimum (see FIG. 13).

The length of the spiral cable is not necessarily set to the length described in JP 2006-176102 A. The following problems occur in the case where the length of an actual spiral cable is shorter than that described in JP 2006-176102 A. In the following description, the steering angle ratio is defined as the steered angle divided by the steering angle.

When the steering limiting process is performed in response to the steering angle approaching its upper limit value that is determined by the length of the spiral cable, the situation is avoided where the steering is operated so as to increase the steering angle to a value larger than its upper limit value. In this case, however, if the steering angle ratio is high, the steering limiting process may not be performed even when the rack shaft contacts the rack stopper, because the steering angle has not reached its upper limit.

On the other hand, in the case where the steering limiting process is performed based on the steered angle, the steering limiting process can be performed before the rack shaft contacts the rack stopper, even if the steering angle ratio is high. In this case, however, if the steering angle ratio is low, the steering angle may reach its upper limit value that is determined by the length of the spiral cable, even if the steered angle has not reached its upper limit. In this case, the steering limiting process may not be performed even if the steering angle reaches its upper limit value.

Not only in the steering devices including a spiral cable and a rack and pinion mechanism, but also in the steering devices in which the steered angle and the steering angle have separate upper limit values, the operation of the steering which causes the steered angle or the steering angle to approach its upper limit value and to exceed its upper limit value may not be able to be limited.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a steering control device that can restrain a steering from being operated to cause a steered angle to exceed its upper limit value and can also restrain the steering from being operated to cause a steering angle to exceed its upper limit value.

According to one aspect of the present invention, a steering control device that outputs an operation signal to a steering device in which a steering angle ratio is variable, the steering angle ratio being a ratio of a steered angle of a steered wheel of a vehicle to a steering angle that is a rotation angle of a steering. The steering control device includes: a steering angle obtaining circuit that obtains the steering angle; a steered angle obtaining circuit that obtains the steered angle; and a steering limiting circuit that performs limiting control to urge a user to stop operating the steering to a side where the steering angle further increases when a disjunction of a condition that the obtained steered angle reaches a steered angle threshold and a condition that the obtained steering angle reaches a steering angle threshold is true.

In the above configuration, when the disjunction of the condition that the obtained steered angle reaches the steered angle threshold and the condition that the obtained steering angle reaches the steering angle threshold is true, the steering limiting circuit performs the limiting control to urge the user to stop operating the steering to the side where the steering angle further increases. Before the steering angle reaches its upper limit value, the steering angle reaches the steering angle threshold and the limiting control can be performed. Moreover, before the steered angle reaches its upper limit value, the steered angle reaches the steered angle threshold and the limiting control can be performed. This can restrain the steering from being operated to cause the steered angle to exceed its upper limit value and can also restrain the steering from being operated to cause the steering angle to exceed its upper limit value

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a system configuration diagram of a steering device including a steering control device according to a first embodiment;

FIG. 2 is a block diagram illustrating a process that is performed by the control device according to the first embodiment;

FIG. 3 is a graph showing thresholds of a steering angle and a steered angle according to the first embodiment;

FIG. 4 is a flowchart illustrating a process that is performed by a maximum value selection circuit;

FIG. 5 is a system configuration diagram of a steering device including a steering control device according to a second embodiment; and

FIG. 6 is a block diagram illustrating a process that is performed by the control device according to the second embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

A first embodiment of a steering control device of the present invention will be described below with reference to the accompanying drawings. As shown in FIG. 1, in a steering device of the present embodiment, a steering wheel 10 that receives input for a steering (steering mechanism) is connected to a reaction force actuator 20 that applies a reaction force, or a force against operation of the steering wheel 10. The reaction force actuator 20 includes a steering shaft 22 fixed to the steering wheel 10, a reaction force-side reduction gear 24, a reaction force motor 26 having a rotary shaft 26 a coupled to the reaction force-side reduction gear 24, and an inverter 28 that drives the reaction force motor 26.

The steering shaft 22 can be coupled to a pinion shaft 42 of a steering actuator 40 via a clutch 12. The steering actuator 40 includes a first rack and pinion mechanism 48, a second rack and pinion mechanism 52, a steered-side motor 56, and an inverter 58.

The first rack and pinion mechanism 48 includes a rack shaft 46 and the pinion shaft 42. The rack shaft 46 and the pinion shaft 42 are arranged at a predetermined crossing angle, and first rack teeth 46 a formed on the rack shaft 46 mesh with pinion teeth 42 a formed on the pinion shaft 42. Steered wheels 30 are coupled to both ends of the rack shaft 46 via tie rods. The steering (steering mechanism) refers to an entire mechanism that converts the input received by the steering wheel 10 into a turning operation of the steered wheels 30.

The second rack and pinion mechanism 52 includes the rack shaft 46 and a pinion shaft 50. The rack shaft 46 and the pinion shaft 50 are arranged at a predetermined crossing angle, and second rack teeth 46 b formed on the rack shaft 46 mesh with pinion teeth 50 a formed on the pinion shaft 50.

The pinion shaft 50 is connected to a rotary shaft 56 a of the steered-side motor 56 via a steered-side reduction gear 54. The inverter 58 is connected to the steered-side motor 56. The rack shaft 46 is accommodated in a rack housing 44.

A steering control device (control device 60) controls steering of the steered wheels 30 in accordance with the operation of the steering wheel 10 by operating the steering device including the reaction force actuator 20 and the steering actuator 40.

In the present embodiment, a steer-by-wire system is thus implemented by the reaction force actuator 20 and the steering actuator 40. The control device 60 normally controls steering of the steered wheels 30 in accordance with the operation of the steering wheel 10 while keeping the clutch 12 disengaged. At this time, the control device 60 obtains a rotation angle θs0 of the rotary shaft 26 a of the reaction force motor 26, torque Trq applied to the steering shaft 22, a rotation angle θt0 of the rotary shaft 56 a of the steered-side motor 56, and a vehicle speed V. The rotation angle θs0 of the rotary shaft 26 a is detected by a steering-side sensor 62. The torque Trq is detected by a torque sensor 64. The rotation angle θt0 of the rotary shaft 56 a is detected by a steered-side sensor 66. The vehicle speed V is detected by a vehicle speed sensor 68.

A spiral cable device 70 is coupled to the steering wheel 10. The spiral cable device 70 includes a first housing 72, a second housing 74, a tubular member 76, and a spiral cable 78. The first housing 72 is fixed to the steering wheel 10, and the second housing 74 is fixed to a vehicle body, The tubular member 76 is accommodated in the space defined by the first and second housings 72, 74 and is fixed to the second housing 74. The spiral cable 78 is wound around the tubular member 76. The tubular member 76 has the steering shaft 22 inserted therein. The spiral cable 78 is electrical wiring that connects a horn 80 fixed to the steering wheel 10 to a battery 82 fixed to the vehicle body, etc.

FIG. 2 shows a part of a process that is performed by the control device 60. An integration circuit M2 converts the rotation angle θs0 detected by the steering-side sensor 62 and the rotation angle θt0 detected by the steered-side sensor 66 to values in an angle region larger than 0 to 360° and outputs rotation angles θs, θt. For example, the rotary shaft 26 a makes a plurality of rotations in the case where the steering wheel 10 is rotated to the maximum to the right or to the left from a neutral position, or a position where the vehicle is moved straight. Accordingly, for example, the integration circuit M2 outputs a rotation angle θs of 720° in the case where the rotary shaft 26 a makes two rotations in a predetermined direction from the state where the steering wheel 10 is in the neutral position. The integration circuit M2 outputs zero when the steering wheel 10 is in the neutral position.

A unit-of-measurement setting unit M4 multiplies the output value of the integration circuit M2, which has been obtained from the output of the steering-side sensor 62, by a conversion factor Ks to calculate a steering angle θh. The unit-of-measurement setting unit M4 also multiplies the output value of the integration circuit M2, which has been obtained from the output of the steered-side sensor 66, by a conversion factor Kt, to calculate a steered angle θp. The conversion factor Ks is determined according to the rotational speed ratio between the reaction force-side reduction gear 24 and the rotary shaft 26 a of the reaction force motor 26. The conversion factor Ks is used to convert the amount of change in rotation angle θs to the amount of rotation of the steering wheel 10. The steering angle θh is therefore the rotation angle of the steering wheel 10 with respect to the neutral position. The conversion factor Kt is the product of the rotational speed ratio between the steering-side reduction gear 54 and the rotary shaft 56 a of the steered-side motor 56, the rotational speed ratio between the pinion shaft 50 and the rack shaft 46, and the rotational speed ratio between the rack shaft 46 and the pinion shaft 42. The conversion factor Kt is used to convert the amount of rotation of the rotary shaft 56 a to the amount of rotation of the steering wheel 10 which is based on the assumption that the clutch 12 is in an engaged state.

In the process of FIG. 2, the rotation angles θs, θt, the steering angle θh, and the steered angle θp are positive when they are rotation angles in the predetermined direction, and are negative when they are rotation angles in the opposite direction. For example, in the case where the rotary shaft 26 a is rotated in the opposite direction to the predetermined direction from the state where the steering wheel 10 is in the neutral position, the integration circuit M2 outputs a negative value. However, this is merely an example of the logic of the control system. In particular, as used herein, the expression “the rotation angles θs, θt, the steering angle θh, and the steered angle θp are large” means that a variation from the neutral position is large. In other words, the expression “the rotation angles θs, θt, the steering angle θh, and the steered angle θp are large” means that the absolute values of the parameters that can have positive and negative values are large.

A limiting reaction force setting circuit M6 sets a limiting reaction force Fie, or a reaction force that acts against further operation of the steering wheel 10 toward its upper limit value when the amount of rotation of the steering wheel 10 approaches its upper limit value.

A base reaction force setting circuit MS sets a base reaction force Fib in accordance with operation of the steering wheel 10. A hysteresis reaction force setting circuit M10 sets a hysteresis reaction force Fih so that the reaction force to be applied to the steering wheel 10 varies depending on the sign of the rate of change of the steering wheel 10. For example, in the case where the steering wheel 10 is being turned to the right with respect to the neutral position, the reaction force to be applied to the steering wheel 10 varies between the case where the steering wheel 10 is turned further to the right and the case where the steering wheel 10 is turned back toward the neutral position.

An addition circuit M12 calculates a total reaction force Fir by adding the limiting reaction force Fie, the base reaction force Fib, and the hysteresis reaction force Fih.

An assist axial force setting circuit M20 calculates an assist axial force Fa based on the torque Trq detected by the torque sensor 64. The assist axial force Fa is set to a larger value as the torque Trq is larger.

A target steering angle setting circuit M22 sets a target steering angle θh* based on the torque Trq detected by the torque sensor 64, the assist axial force Fa, and the total reaction force Fir. The target steering angle setting circuit M22 herein uses a model formula given by the following formula (c1) that correlates a final axial force Ff, which is the sum of the torque Trq and the assist axial force Fa minus the total reaction force Fir, with the target steering angle θh*.

Ff=C·θh*′+J·θh*″  (c1)

The model given by the formula (c1) is a model that defines the relationship between the torque and the rotation angle of the rotary shaft that rotates with rotation of the steering wheel 10 in the configuration in which the steering wheel 10 is mechanically coupled to the steered wheels 30. In the formula (c1), a coefficient of viscosity C models friction of the steering device etc., and a coefficient of inertia J models inertia of the steering device. The coefficient of viscosity C and the coefficient of inertia J are set variably according to the vehicle speed V. θh*′ represents the first-order differentiation of θh* with respect to time, and θh*″ represents the second-order differentiation of θh* with respect to time.

A steering angle control circuit M24 sets, as a manipulated variable for feedback controlling the steering angle θh to the target steering angle θh*, reaction torque Trqa* to be generated by the reaction force motor 26. Specifically, the steering angle control circuit M24 sets the reaction torque Trqa* to the sum of output values of proportional, integral, and derivative elements each receiving the target steering angle θh* minus the steering angle θh.

An operation signal generation circuit M26 generates an operation signal MSs for the inverter 28 based on the reaction torque Trqa* and outputs the operation signal MSs to the inverter 28. For example, this can be implemented by well-known current feedback control in which a command value for a q-axis current is set based on the reaction torque Trqa* and d- and q-axis voltage command values are set as manipulated variables for feedback controlling d- and q-axis currents to their command values. The d-axis current may be controlled to zero. However, in the case where the rotational speed of the reaction force motor 26 is high, the absolute value of the d-axis current may be set to a value larger than zero to perform field weakening control. The absolute value of the d-axis current may be set to a value larger than zero even in a low rotational speed region.

A steering angle ratio variable setting circuit M28 sets a target operating angle θa* for variably setting the steering angle ratio, or the ratio of the steered angle θp to the steering angle θh, based on the vehicle speed V. An addition circuit M30 calculates a target steered angle θp* by adding the target operating angle θa* to the target steering angle θh*.

A steered angle control circuit M32 sets, as a manipulated variable for feedback controlling the steered angle θp to the target steered angle θp*, torque Trqt* to be generated by the steered-side motor 56. Specifically, the steered angle control circuit M32 sets the torque Trqt* to the sum of output values of proportional, integral, and derivative elements each receiving the target steered angle θp* minus the steered angle θp.

An operation signal generation circuit M34 generates an operation signal MSt for the inverter 58 based on the torque Trqt* and outputs the operation signal MSt to the inverter 58. The operation signal generation circuit M34 can generate the operation signal MSt in a manner similar to that of the operation signal generation circuit M26.

The base reaction force setting circuit M8 and the hysteresis reaction force setting circuit M10 receive the target steering angle θh*. Specifically, the base reaction force setting circuit M8 sets the base reaction force Fib to a larger value as the magnitude of the target steering angle θh* (the amount of rotation with respect to the neutral position) is larger. FIG. 2 shows that the base reaction force Fib increases as the target steering angle θh* increases from zero in a predetermined rotational direction. However, the base reaction force Fib increases even if the target steering angle θh* increases in the opposite direction to the predetermined rotational direction.

The limiting reaction force setting circuit M6 receives an end angle θe, or a maximum value of the target steering angle θh* and the target steered angle θp*, and sets the limiting reaction force Fie. The limiting reaction force Fie is set in order to perform the control to increase the force that is applied to the steering wheel 10 both immediately before an end of the rack shaft 46 contacts the rack housing 44 as a result of axial displacement of the rack shaft 46 and immediately before the steering wheel 10 is rotated to its upper limit value determined by the spiral cable 78. This will be described below.

FIG. 3 shows the relationship between respective upper limit values θhH, θpH of the steering angle θh and the steered angle θp. As shown in the figure, in the present embodiment, the upper limit value θhH of the steering angle θh is substantially equal to the upper limit value θpH of the steered angle θp. This is implemented by setting the unit of measurement of the steering angle θh and the steered angle θp by the unit-of-measurement setting unit M4. In the present embodiment, the length of the spiral cable 78 has a small margin so that the steering wheel 10 can still be rotated slightly even after the rack shaft 46 contacts the rack housing 44 as a result of its axial displacement in the case where the clutch 12 is in an engaged state. Since the unit-of-measurement setting unit M4 calculates the steering angle θh as the rotation angle of the steering wheel 10 and calculates the steered angle θp as the rotation angle of the steering wheel 10 which is based on the assumption that the target operating angle θa* is zero, the upper limit value θhH of the steering angle θh is substantially equal to the upper limit value θpH of the steered angle θp.

Accordingly, in the present embodiment, a common threshold θen is used for the steering angle θh and the steered angle θp, and the control to increase the reaction force to the steering wheel 10 is performed before the steering angle θh reaches its upper limit value θhH and the steered angle θp reaches its upper limit value θpH. The limiting reaction force setting circuit M6 shown in FIG. 2 has a map that defines the relationship between the end angle θe and the limiting reaction force Fie. According to this map, the limiting reaction force Fie is set to a positive value in the case where the end angle θe is equal to or larger than the common threshold θen. In particular, if the end angle θe has a relatively large value larger than the common threshold θen, the limiting reaction force Fie is set to such a large value that the steering wheel 10 cannot be operated any further by the user's force. FIG. 2 shows that the limiting reaction force Fie increases as the end angle θe increases from zero in the predetermined rotational direction. However, like the base reaction force Fib, the limiting reaction force Fie increases even if the end angle θe increases in the opposite direction to the predetermined rotational direction.

Specifically, as shown in FIG. 2, the control device 60 includes a maximum value selection circuit M36 that selects a maximum value of the target steering angle θh* and the target steered angle θp*. FIG. 4 shows a process that is performed by the maximum value selection circuit M36. For example, this process is repeatedly performed at predetermined intervals by the maximum value selection circuit M36.

In the series of steps shown in FIG. 4, the maximum value selection circuit M36 first obtains the target steering angle θh* (S10). Next, the maximum value selection circuit M36 obtains the target steered angle θp* (S12). The maximum value selection circuit M36 then outputs a maximum value of the target steering angle θh* and the target steered angle θp* as an end angle θe (S14). As used herein, the “maximum value” means one of the target steering angle θh* and the target steered angle θp* which is farther away from the value (zero) corresponding to the neutral position.

The maximum value selection circuit M36 terminates this series of steps when step S14 is completed.

Functions of the present embodiment will be described.

The target steering angle setting circuit M22 sets the target steering angle θh* in accordance with the torque Trq that is applied by the user to the steering wheel 10. The addition circuit M30 calculates the target steered angle θp* in accordance with the target steering angle θh*. The steered angle control circuit M32 sets the torque Trqt* for controlling the steered angle θp to the target steered angle θp*. The torque of the steered-side motor 56 is thus controlled to the torque Trqt*, so that the steered angle Op is controlled to the target steered angle θp*.

The steering angle control circuit M24 sets the reaction torque Trqa* for controlling the steering angle θh to the target steering angle θh*. The target steering angle θh* is set based on the final axial force Ff by which the sum of the torque Trq and the assist axial force Fa is brought equal to the total reaction force Fir. The base reaction force Fib is set to a larger value as the steering angle θh is larger. Such a target steering angle θh* is set that makes the sum of the torque Trq that is applied by the user to the steering wheel 10 and the assist axial force Fa equal to the total reaction force Fir. Accordingly, the larger the steering angle θh is, the larger the reaction force, namely the torque that is applied by the reaction force motor 26 to turn the steering wheel 10 back toward the neutral position, is.

If the end angle θe is about to increase beyond the common threshold θen, the limiting reaction force Fie increases rapidly as compared to the base reaction force Fib etc. In this case, the total reaction force Fir increases rapidly, and the sum of the torque

Trq and the assist axial force Fa also increases rapidly. As a result, larger torque (reaction torque Trqa*) is generated by the reaction force motor 26 in order to turn the steering wheel 10 back toward the neutral position, which makes it difficult for the user to turn the steering wheel 10 any further. In the case where the end angle θe is the steering angle θh, the steering angle θh is thus sufficiently restrained from changing rapidly and reaching its upper limit value θhH.

When an increase in target steering angle θh* is limited, an increase in target steered angle θp* is also limited. In the case where the end angle θe is the steered angle θp, the steered angle θp is thus sufficiently restrained from changing rapidly and reaching its upper limit value θpH.

The present embodiment described above has the following effects.

(1) The limiting reaction force Fie is set to a value larger than zero in the case where the end angle θe is equal to or larger than the common threshold θen. This can restrain the steering wheel 10 from being operated to cause the steered angle θp to exceed its upper limit value θpH and can also restrain the steering wheel 10 from being operated to cause the steering angle θh to exceed its upper limit value θhH.

(2) The target steering angle setting circuit M22 is formed by a model. This can reduce the workload for calibration of a controller that controls the reaction force to be applied to the steering wheel 10. For example, if the limiting reaction force setting circuit M6 is formed by a map that defines the relationship between the end angle θe and the vehicle speed V and the limiting reaction force Fie, rather than by a model, the amount of calibration data for the limiting reaction force setting circuit M6 is larger than that in the configuration of FIG. 2 because the map has a larger number of dimensions.

(3) The present embodiment includes the spiral cable device 70. If the control to increase the reaction torque is not performed when the steering angle θh becomes equal to or larger than its upper limit value θhH, the torque that increases the steering angle θh is applied to the steering wheel 10 even after the steering angle θh reaches its upper limit value θhH determined by the length of the spiral cable 78. In this case, a force that stretches the spiral cable 78 continues to be applied to the spiral cable 78 even after the spiral cable 78 is stretched to the maximum. This may affect the reliability of the spiral cable 78. In the present embodiment, however, the control to increase the reaction force is performed when the steering angle θh becomes equal to or larger than the common threshold θen. This can restrain a force that stretches the spiral cable 78 from being continuously applied to the spiral cable 78 even after the spiral cable 78 is stretched to the maximum. Reduction in reliability of the spiral cable 78 can therefore be restrained.

A second embodiment of the steering control device will be described below with reference to the drawings. The second embodiment is mainly described with respect to the differences from the first embodiment.

FIG. 5 shows a system configuration according to the present embodiment. In FIG. 5, the members corresponding to those in FIG. 1 are denoted by the same reference characters for convenience. A steering device according to the present embodiment includes a variable steering angle ratio (VGR) system including a variable steering angle ratio (VGR) actuator 90 instead of the reaction force actuator 20. The VGR actuator 90 includes a housing 92, a VGR motor 94, an inverter 100, and a reduction gear mechanism 96. The housing 92 is coupled to the steering shaft 22 so that it can rotate with the steering shaft 22. The VGR motor 94 is accommodated in the housing 92. The reduction gear mechanism 96 is formed by a mechanism comprised of three rotary elements that can rotate differentially with respect to each other. For example, the reduction gear mechanism 96 is formed by a planetary gear mechanism, strain wave gearing, etc. The three rotary elements of the reduction gear mechanism 96 are coupled to the housing 92, a rotary shaft 94 a coupled to a rotary shaft of the VGR motor 94, and an output shaft 98 coupled to the pinion shaft 42, respectively. In the reduction gear mechanism 96, the rotational speed of the output shaft 98 is thus uniquely determined by the rotational speed of the housing 92 and the rotational speed of the VGR motor 94. The VGR actuator 90 adds rotation of the rotary shaft 94 a of the VGR motor 94 to rotation of the steering shaft 22 associated with the steering operation and transmits the resultant rotation to the output shaft 98 through the reduction gear mechanism 96. The VGR actuator 90 thus changes the rotation angle of the output shaft 98 relative to the steering shaft 22. The steering angle ratio, or the ratio of the steered angle θp to the steering angle θh, is thus set variably. As used herein, term “adding” includes both addition and subtraction. In the following description, the rotation angle of the output shaft 98 relative to the steering shaft 22 is referred to as the operating angle θa of the output shaft 98.

A steering angle ratio-side sensor 102 detects the rotation angle θm of the rotary shaft 94 a of the VGR motor 94. The torque sensor 64 detects torque Trq of the output shaft 98. FIG. 6 shows a process that is performed by the control device 60 according to the present embodiment, In FIG. 6, the steps corresponding to those in FIG. 2 are denoted by the same reference characters for convenience.

An operating angle calculation circuit M40 calculates an actual operating angle θa of the output shaft 98 from the rotation angle θm, based on the reduction ratio determined in accordance with the gear ratio between the rotary elements of the reduction gear mechanism 96. A steering angle ratio control circuit M44 calculates steering angle ratio torque Trqv* as a manipulated variable for feedback controlling the operating angle θa to a target operating angle θa*. Specifically, the steering angle ratio control circuit M44 sets the steering angle ratio torque Trqv* to the sum of output values of proportional, integral, and derivative elements each receiving the target operating angle θa* minus the operating angle θa.

An operation signal generation circuit M46 generates an operation signal MSv for the inverter 100, and outputs the operation signal MSv to the inverter 100. The operation signal MSv is a signal for controlling the torque of the VGR motor 94 to the steering angle ratio torque Trqv*. The operation signal generation circuit M46 can generate the operation signal MSv in a manner similar to that of the operation signal generation circuit M26.

A base value setting circuit M48 sets a base value Ta1* of assist torque based on the torque Trq detected by the torque sensor 64 and the vehicle speed V detected by the vehicle speed sensor 68. The base value Ta1* is set to a larger value as the torque Trq is larger.

A subtraction circuit M50 calculates the steering angle θ by subtracting the operating angle θa from the steered angle θp. A correction circuit M54 subtracts the limiting reaction force Fie from the base value Ta1* and outputs the subtraction result as assist torque Ta*.

An operation signal generation circuit M34 generates an operation signal

MSt for controlling the torque of the steered-side motor 56 to the assist torque Ta*, and outputs the operation signal MSt to the inverter 58. The operation signal generation circuit M34 can generate the operation signal MSt in a manner similar to that of the operation signal generation circuit M26.

In the present embodiment, a maximum value selection circuit M36 obtains the steering angle θh in step S10 of FIG. 4, obtains the steered angle θp in step S12, and selects the maximum value of the steering angle θh and the steered angle θp in step S14 to output the selected maximum value as an end angle θe.

Functions of the present embodiment will be described, When the user operates the steering wheel 10, the torque applied to the steering wheel 10 is transmitted to the rack shaft 46 via the output shaft 98. The steered-side motor 56 generates torque in accordance with the assist torque Ta*. This torque is also transmitted to the rack shaft 46.

When the end angle θe becomes equal to or larger than the common threshold θen, the assist torque Ta* becomes smaller than the base value Ta1* by the limiting reaction force Fie. The limiting reaction force Fie increases rapidly once the end angle θe exceeds the common threshold θen. Accordingly, the assist torque Ta* decreases rapidly once the end angle θe exceeds the common threshold θen. Large torque is therefore required in order to operate the steering wheel 10 to further increase the steering angle θh.

At least one of the matters described in each of the above embodiments may be modified as follows. The steering angle obtaining circuit described in “SUMMARY OF THE INVENTION” corresponds to the control device 60 that performs step S10. The steering angle obtaining circuit corresponds to the control device 60 that performs step S12. The steering limiting circuit corresponds to the limiting reaction force setting circuit M6, the target steering angle setting circuit M22, the steering angle control circuit M24, the operation signal generation circuit M26, the maximum value selection circuit M36, etc, in the first embodiment, and corresponds to the limiting reaction force setting circuit M6, the maximum value selection circuit M36, the correction circuit M54, the operation signal generation circuit M34, etc. in the second embodiment. The external device corresponds to the battery 82.

In the above embodiment, the target steering angle setting circuit (M22) sets the target steering angle θh* by using the model formula given by the formula (c1). However, the present invention is not limited to this. For example, the target steering angle setting circuit (M22) may set the target steering angle θh* by using a model formula given by the following formula (c2).

Ff=K·θh*+C·θh*′+J·θh*″  (c2)

A spring coefficient K models the influence of the vehicle and is determined by specifications such as suspension and wheel alignment.

The present invention is not limited to setting the target steering angle θh* by using a model formula. For example, the target steering angle θh* may be set by using a map that defines the relationship between the final axial force Ff and the vehicle speed V and the target steering angle θh*. Regarding the reaction force, in the first embodiment, the control device 60 may not include the base reaction force setting circuit M8 and the hysteresis reaction force setting circuit M10. That is, the total reaction force Fir may not include the base reaction force Fib and the hysteresis reaction force Fih.

In the above configuration, the target steered angle setting circuit (M22, M28, M30) calculates the target steered angle θp* by adding the target operating angle θa* to the target steering angle θh* calculated based on the final axial force Ff. However, the present invention is not limited to this. For example, the target steered angle setting circuit (M22, M28, M30) may directly calculate the target steered angle θp* by using a model formula based on the above formula (c1) or (c2).

In the above embodiment, the steering angle control circuit (M24) sets the manipulated variable for feedback controlling the steering angle θh to the target steering angle θh* to the sum of the output values of the proportional, integral, and derivative elements each receiving the target steering angle θh* minus the steering angle θh. However, the present invention is not limited to this. For example, the steering angle control circuit M24 may be formed only by the integral element and may set the manipulated variable to the output value of the integral element. Alternatively, the steering angle control circuit M24 may be formed only by the integral and proportional elements and may set the manipulated variable to the sum of the output values of the integral and proportional elements.

The manipulated variable is not limited to torque. For example, the manipulated variable may be a q-axis current in the case of controlling a d-axis current of the reaction force motor 26 to zero. In the above embodiment, the steered angle control circuit (M32) sets the manipulated variable for feedback controlling the steered angle θp to the target steered angle θp* to the sum of the output values of the proportional, integral, and derivative elements each receiving the target steered angle θp* minus the steered angle θp. However, the present invention is not limited to this. For example, the steered angle control circuit M32 may be formed only by the integral element and may set the manipulated variable to the output value of the integral element. Alternatively, the steered angle control circuit M32 may be formed only by the integral and proportional elements and may set the manipulated variable to the sum of the output values of the integral and proportional elements.

The manipulated variable is not limited to torque. For example, the manipulated variable may be a q-axis current in the case of controlling a d-axis current of the steered-side motor 56 to zero. The process that is performed by the unit-of-measurement setting circuit (M4) is not limited to converting the steering angle to the rotation angle of the steering wheel 10, and converting the steered angle to the rotation angle of the steering wheel 10 in the state where the target operating angle θa* is zero. For example, the unit-of-measurement setting circuit (M4) may convert the steered angle to the rotation angle of the rotary shaft 26 a in the state where the target operating angle θa* is zero. The rotation angle herein includes angles smaller than 0° and larger than 360°. Alternatively, for example, the unit-of-measurement setting circuit (M4) may convert the steering angle to the rotation angle of the rotary shaft 56 a in the state where the target operating angle θa* is zero. The rotation angle herein includes angles smaller than 0° and larger than 360°.

For example, the steering angle may be θs·θtH and the steered angle may be θt·θsH based on the upper limit value θsH of the rotation angle θs and the upper limit value θtH of the rotation angle θt. In this case, the common threshold is θsH·θtH.

The unit of measurement of the steering angle and the steered angle need not necessarily be the unit of measurement for the rotation angle. For example, as described in the description regarding the steered-side sensor, in the configuration in which the steered-side sensor detects the amount of axial displacement of the rack shaft 46, the steering angle may be a value obtained by converting the rotation angle θs to the amount of displacement of the rack shaft 46 in the state where the target operating angle θa* is zero.

In the first embodiment, the maximum value (θen) is the maximum value of the target steering angle θh* and the target steered angle θp*. However, the maximum value (θen) may be a maximum value of the steering angle θh and the steered angle θp.

The maximum value (θen) is not limited to the larger one of the two parameters, namely the steering angle θh or the target steering angle θh* and the steered angle θp or the target steered angle θp*. For example, in the configuration in which different steered-side motors apply torque to different portions of the rack shaft 46, the maximum value (θen) may be a maximum value of the steering angle and a pair of steered angles in accordance with the rotation angles detected by these steered-side motors. For example, in four-wheel steering vehicles, the maximum value (θen) may be a maximum value of the steering angle, the steered angle of the front wheels, and the steered angle of the rear wheels.

The steering angle threshold (θen) and the steered angle threshold (θen) are not limited to the common thresholds that are equal to each other. For example, the control device 60 may not include the unit-of-measurement setting circuit M4, and may include, instead of the limiting reaction force setting circuit M6, a process of setting a limiting reaction force based on the rotation angle θs and a process of setting a limiting reaction force based on the rotation angle θt. The control device 60 may set the limiting reaction force Fie to be input to the addition circuit M12 to the sum of the pair of limiting reaction forces or a larger one of the pair of limiting reaction forces.

Regarding the control to increase the reaction force, in the first embodiment, the reaction force actuator 20 is operated in order to feedback control the steering angle Oh to the target steering angle θh*, and the control to increase the reaction force is implemented by setting the target steering angle θh* based on the limiting reaction force Fie. However, the present invention is not limited to this. For example, the control device 60 may include a map that defines the relationship between the end angle θe and the vehicle speed V and the limiting reaction force to calculate the limiting reaction force, and may set a command value of the torque of the reaction force motor 26 to the assist torque minus the limiting reaction force.

In the first embodiment, the control to increase the reaction force may be performed with the clutch 12 switched to the engaged state. This makes it easier to increase the reaction force to be applied to the steering wheel 10 even if the reaction force motor 26 is reduced in size.

The limiting control to operate the reaction force actuator 20 to urge the user to stop operating the steering to the side where the steering angle further increases is not limited to the control to increase the reaction force. For example, the limiting control may be the process of applying vibrations to the steering wheel 10.

The limiting control is not limited to the control using the reaction force actuator 20. For example, the limiting control may be a process of giving an alarm by operating a device other than the steering device which includes a speaker etc. Regarding the process of variably setting the steering angle ratio, the steering angle ratio variable setting circuit M28 variably sets the steering angle ratio in accordance with the vehicle speed V in the above embodiments. However, the present invention is not limited to this. For example, the steering angle ratio may be variably set based on the steering angle θh in addition to the vehicle speed V. For example, the steering angle ratio may be controlled as a manipulated variable for known drive assist control such as electronic stability control and lane keep assist control for vehicles. For example, this can be implemented by obtaining information on the steering angle ratio from a device that calculates the steering angle ratio in accordance with the drive assist control.

The steering-side sensor (62) is not limited to the rotation angle sensor that detects the rotation angle θs0 of the rotary shaft 26 a of the reaction force motor 26. For example, the steering-side sensor may be a sensor that detects the rotation angle of the steering shaft 22.

The steered-side sensor (66) is not limited to the rotation angle sensor that detects the rotation angle of the rotary shaft 56 a of the steered-side motor 56. For example, the steered-side sensor may be a sensor that detects the rotation angle of the pinion shaft 50. The steered-side sensor is not limited to the rotation angle sensors. For example, the steered-side sensor may be a sensor that detects the amount of axial displacement of the rack shaft 46.

The steering actuator (40) is not limited to the steering actuator including the first rack and pinion mechanism 48 and the second rack and pinion mechanism 52. For example, the steering actuator may apply the torque of the steered-side motor 56 to the pinion shaft 42 of the first rack and pinion mechanism 48 via the steered-side reduction gear 54.

The present invention is not limited to the configuration including the rack and pinion mechanisms. For example, the present invention may be of a ball-nut type. Control of the VGR system is not limited to the control shown in FIG. 6. For example, the target steered angle θp* may be calculated from the sum of the base value Ta1* and the torque Trq based on a model, the assist torque Ta* may be corrected based on a manipulated variable for feedback controlling an actual steered angle θp to the target steered angle θp*, and the corrected assist torque Ta* may be applied to the operation signal generation circuit M34.

The steering-side device (80) is not limited to the horn 80. The steering-side device (80) may be an airbag etc.

The upper limit value of the steering angle need not necessarily be determined by the spiral cable. For example, in the case where the steering device includes a sensor that detects the rotation angle of the steering shaft 22 and the sensor has a detection upper limit value, the upper limit value of the steering angle may be determined by the detection upper limit value of the sensor.

The upper limit value of the steered angle need not necessarily be determined by the upper limit value of the amount of displacement of the rack shaft 46. For example, in the case where the steering device includes a sensor that detects the amount of displacement of the rack shaft 46 in order to detect the steered angle and the sensor has a detection upper limit value, the upper limit value of the steered angle may be determined by the detection upper limit value of the sensor. 

What is claimed is:
 1. A steering control device that outputs an operation signal to a steering device in which a steering angle ratio is variable, the steering angle ratio being a ratio of a steered angle of a steered wheel of a vehicle to a steering angle that is a rotation angle of a steering, the steering control device comprising: a steering angle obtaining circuit that obtains the steering angle; a steered angle obtaining circuit that obtains the steered angle; and a steering limiting circuit that performs limiting control to urge a user to stop operating the steering to a side where the steering angle further increases when a disjunction of a condition that the obtained steered angle reaches a steered angle threshold and a condition that the obtained steering angle reaches a steering angle threshold is true.
 2. The steering control device according to claim 1, further comprising: a unit-of-measurement setting circuit that sets a unit of measurement of the steering angle obtained by the steering angle obtaining circuit and the steered angle obtained by the steered angle obtaining circuit so that the steered angle threshold and the steering angle threshold are common thresholds that are equal to each other, wherein the steering limiting circuit performs the limiting control when a maximum value of the steered angle and the steering angle is equal to or larger than the common threshold.
 3. The steering control device according to claim 1, wherein the limiting control is control to increase a reaction force by outputting an operation signal to the steering device, the reaction force being a force against operation of the steering.
 4. The steering control device according to claim 3, wherein the steering device includes a reaction force actuator that applies to the steering the reaction force that is the force against operation of the steering, and a steering actuator that applies a force that steers the steered wheel at least with power transmission being cut off between the steered wheel and the steering, and the steering limiting circuit performs the control to increase the reaction force by outputting an operation signal to the reaction force actuator.
 5. The steering control device according to claim 2, wherein the steering device includes a reaction force actuator that applies to the steering a reaction force that is a force against operation of the steering, and a steering actuator that applies a force that steers the steered wheel at least with power transmission being cut off between the steered wheel and the steering, the steering limiting circuit includes a limiting reaction force setting circuit that sets a limiting reaction force that is a reaction force for the limiting control, to a value larger than zero if the maximum value of the steered angle and the steering angle is equal to or larger than the common threshold, a target steering angle setting circuit that sets a target steering angle that is a target value of the steering angle, based on the set limiting reaction force, a steering angle control circuit that operates the reaction force actuator in order to feedback control the steering angle, which is based on an output value of a steering-side sensor that detects a rotation angle of a rotary shaft that rotates in accordance with rotation of the steering, to the target steering angle, a target steered angle setting circuit that sets a target steered angle that is a target value of the steered angle, based on the set limiting reaction force, and a steered angle control circuit that operates the steering actuator in order to feedback control the steered angle, which is based on an output value of a steered-side sensor that detects an amount of displacement of a member that is displaced in accordance with steering of the steered wheel, to the target steered angle, the unit-of-measurement setting circuit performs a predetermined arithmetic operation on at least one of the output value of the steered-side sensor and the output value of the steering-side sensor to produce at least one of the steering angle to be input to the steering angle control circuit and the steered angle to be input to the steered angle control circuit, the steering angle obtaining circuit obtains the target steering angle, and the steered angle obtaining circuit obtains the target steered angle.
 6. The steering control device according to claim 3, wherein the steering device includes a steering actuator that steers the steered wheel, and a variable steering angle ratio actuator that changes the steering angle ratio with transmitting torque applied to the steering to the steered wheel, and the steering limiting circuit performs the control to increase the reaction force by performing an operation to reduce an assist force that assists, with the steering actuator, steering of the steered wheel which is caused by operation of the steering.
 7. The steering control device according to claim 1, wherein the steering is equipped with a steering-side device that rotates with the steering, and the steering-side device is connected via a signal cable to an external device that rotates relative to the steering. 